1. Field of Invention
The present invention relates to power converters, and more particularly, relates to a synchronous rectifying circuit for a resonant switching power converter.
2. Description of Related Art
FIG. 1 shows a schematic circuit of a conventional resonant switching power converter. The circuit includes a transformer 10 to provide galvanic isolation from a line input to an output of the power converter for safety. Switches 20 and 30 develop a half bridge circuit to switch the transformer 10 and a resonant tank formed by an inductor 5 and a capacitor 40. The inductor 5 can be an inductance device or a leakage inductance of a primary winding NP of the transformer 10. The inductance of the inductor 5 and the capacitance of the capacitor 40 determine a resonant frequency f0 of the resonant tank as following equation shows:
                              f          0                =                  1                      2            ⁢                                                  ⁢            π            ⁢                                          L                ×                C                                                                        (        1        )            
where L and C are respectively the inductance of the inductor 5 and the capacitance of the capacitor 40.
The transformer 10 transfers the energy from the primary winding NP to secondary windings NS1 and NS2 of the transformer 10. Rectifiers 61, 62 and a capacitor 65 perform the rectifying and filtering for generating a DC voltage VO at the output of the power converter. The detailed skill of the resonant switching power converter can be found in the text book “Resonant Power Converters” authored by Marian K. Kazimierczuk and Dariusz Czarkowski in 1995 and published by John Wiley & Sons, Inc.
Although resonant switching power converters can achieve high efficiency and low electric-magnetic interference (EMI), a forward voltage of the rectifiers 61 and 62 still causes significant power losses. The objective of the present invention is to provide a synchronous rectifying circuit for resonant switching power converters to achieve higher efficiency.